Hair dyes can be classified by the dyes to be used or by whether they have any bleaching action on melanin. Typical examples include a two-part permanent hair dye composed of a first part containing an alkali agent, an oxidation dye and optionally a direct dye such as a nitro dye and a second part containing an oxidizing agent; and a one-part semi-permanent hair dye containing an organic acid or an alkali agent, and a direct dye such as acid dye, basic dye or nitro dye.
The above-described permanent hair dye is however accompanied with the drawbacks that the color tone imparted by an oxidation dye is not so vivid and that the color of the hair dyed with a vivid-color producing nitro dye ordinarily employed as a direct dye markedly fades over time and becomes dull quickly even if the color tone immediately after dyeing is very vivid (Japanese Laid-Open Publication No. Hei 6-271435).
Recently, hair dyes containing a so-called cationic dye have been reported. The cationic dye however involves the drawback that it is unstable against an alkali agent used for an alkali semi-permanent or permanent hair dye, hydrogen peroxide used as an oxidizing agent for a two-part hair dye or a reducing agent added to a permanent hair dye. In the pamphlet of International Patent Application Laid-Open No. 99/07334 (which will hereinafter be called “WO99/07334”), described is a hair dye containing a specific methine type cationic dye stable against an oxidizing agent, a reducing agent or the like. According to the investigation by the present inventor, however, any one of the methine type cationic dyes disclosed specifically in the pamphlet had insufficient stability upon storage within a pH range of 2 to 11, that is, a pH range ordinarily employed for a hair dye. Described specifically, these methine type cationic dyes were particularly unstable against an alkali agent. In a hair dye containing an alkali agent, they decomposed with the passage of time after storage and lost most or their dyeing property. Even when the hair dye was used within an acid to neutral pH range, they gradually decomposed and their dyeing property lowered. In addition, it has been revealed that mixing with a second component part containing an oxidizing agent causes prompt decomposition of them, thereby lowering their dyeing property and in a permanent hair dye containing a reducing agent, their dyeing property is impaired by the storage, thus showing that their stability against an oxidizing agent and a reducing agent is considerably insufficient contrary to the description in the pamphlet.